As well known, the performance of the cutting edge formed on the tip end of an edge tool largely depends on into what shape the ridge (tip) of the cutting edge is processed. In the case of a ridge 11 of a cutting edge shown in FIG. 1(a), as the ridge of the cutting edge is sharpened by polishing or the like as shown in FIG. 1(b), the cutting ability improves, but the mechanical durability tends to deteriorate, such as a chipping 12 being more likely to occur on the ridge of the cutting edge during use as shown in FIG. 1(c).
Furthermore, it is also common practice to perform beveling, such as forming a facet or chamfer (collectively referred to as a facet), in order to increase the strength of the ridge of the cutting edge (see Japanese Patent Application Laid-Open No. 2004-58168, for example). FIG. 1(d) shows an example in which a facet 13 is formed on one surface of a ridge 11′ of the cutting edge shown in FIG. 1(b), and FIG. 1(e) shows an example in which facets 14 are formed on two surfaces of the ridge 11′ of the cutting edge. For example, if the facets 14 are formed on the ridge 11′ of the cutting edge as shown in FIG. 1(e), a chipping or other damage is less likely to occur during use as shown in FIG. 1(f).
It is difficult to form, on the ridge of the cutting edge, a facet with the optimal shape, such as a planer surface or a curved surface, or optimal design values, such as an optimal angle and an optimal width. This is because the error of attachment to a processing apparatus, variations of processing apparatuses or the like in actual use of the edge tool are not known in advance. Even if such use conditions are known in advance, it is still not easy to design an optimal facet.
If the facet shape is not optimized, even if a facet is formed on the ridge of the cutting edge, such as a ridge 15 of a cutting edge with a facet 14 formed thereon shown in FIG. 2(a), a crack 16 occurs on the ridge as shown in FIG. 2(b), and a chipping 17 occurs from the crack 16 as shown in FIG. 2(c). Such chippings occur one after another, leading to a significant problem that the ridge of the cutting edge changes shape as shown in FIG. 2(d), degrades functionality as the edge tool and becomes unusable. Note that the optimal values of the shape of the facet are expected to depend on the kind of the material to be processed and the processing conditions.